The Rust Sea is an area of unstable planetary matter and molecular flux located along Cybertron's equator, stretching from Tyrex to Polyhex. Or in simple terms, it's exactly what its name implies. It's a sea, but it's not full of water.
The Hydrax Plateau rises out of the center of it. Cybertron's primary spaceport is located here, where planetary visitors must check in first.
The sea is plagued by constant "rust storms" that scour the area.
Fall of Cybertron
Ancient Cybertronians constructed a tomb in the area that would one day become the Rust Sea. The tomb was constructed to hold a map room containing the coordinates to various different worlds, including a primitive blue one. They also constructed a Space Bridge in the area.
The Decepticon Shockwave found this tomb and set up his lab here, both for secrecy, as few would brave the violent rust storms common in the area, and for economy, as there was a large (and likely last remaining) lake of raw energon in the area. Shockwave also found both the map room and the space bridge which he replicated and constructed a new space bridge to use in his plans. The last of Shockwave's additions to the are was a refinery to process the raw energon and hold the toxic by-products. The Insecticons also built their "nest" in the refinery.
- In FOC, the rust sea is depicted as more of a desert, with the rust storms being analogous to sand storms, rather than an ocean as depicted in previous fiction.
- This would also make the concept more realistic as rust will eventually degenerate into gritty particles like sand.